Alloy article having a porous outer surface and process of making same



United States Patent 3,190,749 ALLDY ARTICLE HAVING A PORGUS QUEERSURFACE AND PROCESS Gi MAKING SAME Richard A. Fleming, North Tonawanda,N31, assignor to E. I. du Pont de Nemours and Company, Wilmington, Dei.,a corporation of Delaware No Drawing. Filed July 23, 1963, Ser. No.296,900 Claims. (Cl. 75-122) This application is a continuation-in-partof my application Serial No. 58,918, filed September 28, 1960, and nowabandoned.

This invention relates to nickel, platinum, .and palladium alloyarticles having porous surface structures and to methods for preparingsaid article's.

In accordance with the present invention, articles of nickel, platinum,or palladium alloys, especially ferrousnickel alloys andferrous-nickel-chromium stainless steel alloys, can be prepared thathave porous and sponge-like structures useful as catalysts or aslubricant accommodat-ors. Surface porosity is best accomplished byselectively removing or leaching out with a liquid metal or salt meltone component from a multiple component solid alloy. By way of example,in the formation of porous surfaces the preferential dissolution ofcopper in Cu-Ni alloys can be accomplished with liquid Ag at elevatedtemperatures as reported by Harrison and Wagner (Acta. Met. 7, 722[1959]). A related method can be cited in the formation of Raney nickelfrom Ni-Al alloys in which aluminum is dissolved with the aid ofconcentrated NaOH.

The present invention relates to the formation of porous surfaces onalloy articles containing at least one metal selected from the subclassof Group VIII of the Periodic Table consisting of nickel, platinum, andpalladium by the selective removal of a portion of said metal using amelt containing a IIA metal such as calcium, strontium, barium,

and magnesium. It has been found in accordance with the invention thatthe degree of porosity, including the number of interstices per unitarea, depth of interstice penetration and the final composition of thealloy treated can be controlled so that very unique articles may beformed.

The mechanism of thepresent invention is related to liquid-t-o-soliddiffusion in which the relative solubilities of Cr, Ni, Co, Al,-Si, andMn in liquid HA metals and solid ferrous articles are taken advantage ofin the preparation of diffusion alloy coatings. Detailsof this conceptare disclosed in copending application Serial No. 139,369, filedSeptember 20,1961, now abandoned, which is a continuation-in-part ofapplications Serial No. 44,015, filed July 20, 1960, abandoned, andSerial No. 835,171, filed Augustll, 1959, also abandoned. Alloy coatingsform because of the tendency'of the alloying metal to move from theliquid phase to the solid phase. Control is primarily achieved byvarying the concentration of a particular diffusing element in the melt.

It has been found in accordance with the present invention that if ametal article is contacted in a certain temperature range with .a meltof a IIA metal, preferably calcium, wherein the metal is an alloycontaining at least 8% by weight of a metal component which is solublein the melt and a second metal component which is relatively insolublein the melt, unique and -useful articles having controlled surfaceporosity are formed. Metals which may form the soluble metal componentof the alloy article treated are the art recognized subclass of GroupVIII of the Periodic Table consisting of nickel, platinum, andpalladium. Metals which may form the second or insoluble component ofthe alloy article treated are titanium,

zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten,manganese, iron, or cobalt. Nickel is preferred as the metal constitutinthe soluble component and chromium and/ or iron are preferred as themetals constituting the insoluble component of the alloy articletreated. I

A pure HA metal melt promotes the most porosity; increasing the contentof the soluble metal in the melt or decreasing the amount of HA melt byincorporating diluent materialsin the melt reduces the dissolvingtendency of the melt and the depth of porosity as well. As expected,when treated under the same conditions, those articles having thehighest original content of soluble metal component develop the mostsurface porosity. Alloy articles containing from about 880% by weight ofthe soluble metal component and from 92-20% by weight of the insolublemetal component are found to be suitable for treatment in accordancewith the invention to prepare articles having a porous surface. Alloyshaving less than 8% of the soluble metal do not seem to develop theporosity characteristic of articles of the invention and alloys havingmore than 80% of the soluble metal generally develop too extensive adegree of porosity to retain suitable strength for most end uses of theresulting article. More preferably, the proportion of soluble metal inthe starting alloy article is in a range of from about 18-40% by weightwith the insoluble metal component being in the range of from about82-60% by weight.

It is found that to form the articles of the invention,

the contacting method described should be carried out in a temperaturerange of from 1050 C. to 1300 C. At temperatures below 1050 C. littleporosity develops and the time for any porosity to be obtained becomesexcessive. The upper temperature selected for the contacting method ofthe invention will be dependent in large part on the composition of thealloy article treated and the boiling point of the HA metal containingmelt.

Articles prepared in accordance with the present invention arecharacterized by having a porous outer surface containing interstices ofcomparatively uniform dimensions along the grain boundaries extendinginwardly in .a direction normal to the surface. The interstices aresubstan tially circular in cross section with the total crosssectionalarea of the individual interstices being essentially cons-tautalong their inward length so that in their longer dimen sion they areU-shaped rather than V-shaped in configura tion. While the degree ofporosity #(the number of interstices per unitarea) and the depth ofpenetration of the interstices are controlled by the relativeconcentrations of soluble metal in the solid article being treated andthe melt, the individual hole diameters appear to be scarcely affectedby these factors. The interstices have diameters of approximately /6 milplus or minus a factor of 2 3. It is found that the soluble metal ispreferentially depleted in the porous outer surface of the article.Obviously, where the treatment of the invention is practiced on bulkyarticles, the difference in composition of the porous surface article ofthe invention is only slightly different from that of the originalarticle. However, in cases where the treatment of the invention ispracticed on very thin articles, such as alloy foils, the composition ofthe porous surface article of the invention may be appreciablydifferent'frorn that of the original article, although it has been foundthat invariably an appreciable content, usuallynot lower than 1% byweight of the soluble metal, remains present in the treated article.

If desired, chromium and other diffusing elements such as disclosed incopending application Serial No. 139,369 can be simultaneously diffusedinto the article being treated from which the soluble component is beingremoved. This provides a convenient means to maintain the content of theinsoluble component of the article, such as chromium, constant at thesurface or to significantly enrich it in content at the surface orintroduce addi tional alloying elements at the surface of the article.

A better understanding of the invention will be gained from thefollowing description which more clearly illustrates the articles of theinvention and the preferred modes of preparing the same. In thisdescription, all percentages are -by weight unless otherwise noted.

A series of identical panels of Incoloy 2" x /2" x A having alloycompositions of 21.6% chromium, 32.3% nickel, 43.0% iron, and 3.1% tracemetals, were each treated for about an hour in a bath of molten calcium(550 grams) at about 1100 C. The first panel was treated for an hourafter which 20 grams of nickel were added to the bath before the secondpanel was treated. Between each treatment time 20 grams of nickel wereadded to the bath. The bath was maintained at about 1100 C. and thepanels were fastened to a rotating stirrer (about 100 rpm.) which wasimmersed in the bath. In the above manner 6 consecutive panels were eachtreated for one hour and thereafter their surface compositions wereanalyzed by X-ray fluorescence to determine the average composition ofthe top 0.10.2 mil of the panel.

It should be noted that about the same small amount of chromium wasremoved from each sample during treatment and the amount of nickelremoved is greatly dependent on the nickel-calcium ratio of the bath.

The surfaces were examined visually and panels 1 through 5 all hadinterstices at their surfaces; panel No. 6 was virtually unchanged. Themajority of these interstices were associated with grain boundaries andgrain boundary intersections. The number of interstices per unit areadecreased in the order l 2 3 4 5 6.

The panels were then cross sectioned and examined to determine the depthof the affected surface layer.

Depth (mils) of the From the above, it can be seen that the depth of theaffected zone is critically dependent on the bath composition.

The experiments described above show that the amount of porosityproduced by treatment of a given Fe-Cr-Ni solid alloy in a Ca-Ni liquidbath is dependent on the nickel content of the bath. A series of alloysof differing nickel content treated together in the same bath of calcium(or alternatively in Ca-Ni) yielded different degrees and depths ofsurface porosity much as in the aforementioned experiment. This time thebath composition Was fixed and the composition of the solid was varied.Those solid alloys with highest nickel contents developed porosity tothe greatest depth and degree. Nevertheless, it was found in thistesting that porous articles of the invention could be readily formed bytreating type 304 stainless (18-20% Cr, 8l1% Ni, balance essentially Fe)and type 316 stainless (16-18% Cr, -14% Ni, balance essentially Fe) in amelt of calcium.

A reduced chromium content on the sample surface tends to reducecorrosion resistance. Consequently, an experiment was carried out toshow that the chromium 4 content can be increased (or alternativelymaintained constant) while the nickel content is reduced with attendantdevelopment of porosity. The first-mentioned experiment was repeatedwith but one modification. The initial bath contained powdered chromium(40 gm.) in addition to calcium (500 gm.). The sequence of treatments ofstirred Incoloy samples with intervening nickel additions (but nofurther addition of chromium) was carried out as before. Surfaceanalyses are reported below:

Bath Composition Average Surface Comp0sition of Solid Panel Gms. CaGrns. Cr Gms. Ni Percent Percent Percent Cr Ni Fe 500 40 0 71 0. 4Balance 500 40 20 56 6 D0. 500 40 40 48 10 D0. 500 10 00 40 17 Do. 50040 8O 25 Do. 500 100 24 39 D0.

Surface examination again indicated that holes of about /6 mil diameterwere present on all samples except No. 12; the holes were primarilyassociated with grain boundaries. The number of holes per unit areadecreased in the order: 7 8 9 10 11 12. Since chromium is sparinglysoluble in melts of IIA metals, only a small amount of chromium need beincorporated in the melt to favor liquid-to-solid diffusion.

Cross section examinations established the following depths of theaffected areas:

Depth (mils) of the porous Panel: surface layer These'porous-surfacedarticles are extremely resistant to corrosion.

The above examples are illustrative of treating nickel 7 alloys in whichiron is the insoluble metal component.

Nickelalloys composed of nickel and any one or combination of the othermetals mentioned hereinbefore as suitable as an insoluble component canalso be treated to yield articles of the invention having the desiredporous surface. For example, nichrome alloys containing approximately-80% by weight nickel, balance chromium, can be treated to yield highlyporous structures through the selective removal of nickel. The amount ofnickel selectivelyremoved can be controlled by varying the amount ofnickel in the melt. Thus, articles having a porous surface predominantlycomprised of chromium with a small residual amount of nickel can beformed by treating chromium-rich nichrome articles with a melt having acontrolled, low nickel concentration.

Since the stainless steel articles are believed to have particularpractical importance it is contemplated that the insoluble metalstitanium, zirconium, vanadium, tantalum, niobium, molybdenum, tungsten,manganese, and cobalt will usually be included as additional alloyingelements with iron and chromium to make up the second or insoluble metalcomponent of the alloy article to be treated. For instance, an alloycontaining about 20% nickel, 20% cobalt, 21% chromium, 29% iron, 3%molybdenum, 2.5% tungsten, and 4.6% other metals and nonmetals can betreated in a melt of calcium to yield an article of the invention.Obviously, moving machine parts having corrosion-resistant stainlesssteel porous sur faces have obvious utility, particularly in thepreparation of piston rings, heads, bearings, and the like.

Although the above examples relate to nickel alloys,

it is to be appreciated that platinum or palladium alloys wherein thesemetals are alloyed in like manner With an insoluble metal component maybe acted on in the same manner to produce similar articles. Moreover,the above examples are illustrative only of the treatment of bulksubstrates. However, the principles of the present invention can beutilized to treat thin substrates. For instance, alloy foils whentreated in the same manner contain perforations which in some cases canbe made in such number for the treated article to resemble a sieve.

While other modifications of this invention'which may be employed withinthe scope of the invention have not been described, the invention isintended to include all such as may be comprised within the followingclaims.

I claim:

1. An article comprising an alloy consisting essentially .of from up to80% by weight of at least one metal selected from the subclass of GroupVIII of the Periodic Table consisting of nickel, platinum, and palladiumwith the remainder being essentially at least one additional alloyingmetal selected from the group consisting of titanium, zirconcium,vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese,iron, and cobalt; said article being characterized in having a porousouter surface containing interstices along the grain boundariesextending inwardly in a direction normal to the surface, saidinterstices being substantially circular in cross section with the totalcross sectional area of the individual interstices being essentiallyconstant along their inward length.

2. An article comprising an alloy consisting essentially of from up to80% by Weight nickel and the remainder being essentially at least oneadditional alloying metal selected from the group consisting oftitanium, zirconium, vanadium, niobium, tantalum, chromium, molybenum,tungsten, manganese, iron, and cobalt; said article being characterizedin having a porous outer surface containing interstices along the grainboundaries extending inwardly in a direction normal to the surface, saidinterstices being substantially circular in cross section with the totalcross sectional area of the individual interstices being essentiallyconstant along their inward length.

3. An article comprising an alloy consisting essentially of from up toby weight nickel and the remainder being essentially iron and chromium;said article being characterized in having a porous outer surfacecontaining interstices along the grain boundaries extending inwardly ina direction normal to the surface, said interstices being substantiallycircular in cross section with the total cross sectional area of theindividual interstices being essentially constant along their inwardlength.

4. The article of claim 2 in which iron is the alloying metal withnickel.

5. The article of claim 2 in which chromium is the alloying metal withnickel.

6. The method of forming a porous outer surface on an alloy consistingessentially of from 8 to 80% by weight of at least one metal selectedfrom the subclass of Group VIII of the Periodic Table consisting ofnickel, platinum, and palladium and from 92 to 20% of at least onealloying metal selected from the group consisting of titanium,zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten,manganese, iron, and cobalt, comprising contacting said alloy with amelt of 21 HA metal at a temperature of from 1050-1300 C.

7. The method of forming a porous outer surface on an alloy articleconsisting essentially of from 8 to 80% by weight nickel and from 92 to20% by weight of at least one alloying metal selected from the groupconsisting of titanium, zirconium, vanadium, niobium, tantalum,chromium, molybdenum, tungstein, manganese, iron, and cobalt, comprisingcontacting said alloy with a melt of a IIA metal at a temperature offrom 1050-1300 C.

8. The method of claim 7 in which iron and chromium are the alloyingmetals with nickel.

9. The method of claim 7 in which iron is the alloying metal withnickel.

10. The method of claim 7 in which chromium is the alloying metal withnickel.

References Cited by the Examiner UNITED STATES PATENTS 2,740,730 4/56Banus 117l14 DAVID L RECK, Primary Examiner.

2. AN ARTICLE COMPRISING AN ALLOY CONSISTING ESSENTIALLY OF FROM UP TO80% BY WEIGHT NICKEL AND THE REMAINDER BEING ESSENTIALLY AT LEAST ONEADDITIONAL ALLOYING METAL SELECTED FROM THE GROUP CONSISTING OFTITANIUM, ZIRCONIUM, VANADIUM, NIOBIUM, TANTALUM, CHROMIUM, MOLYBENUM,TUNGSTEN, MANGANESE, IRON, AND COBALT; SAID ARTICLE BEING CHARACTERIZEDIN HAVING A POROUS OUTER SURFACE CONTAINING INTERSTICES ALONG THE GRAINBOUNDARIES EXTENDING INWARDLY IN A DIRECTION NORMAL TO THE SURFACE, SAIDINTERSTICES BEING SUBSTANTIALLY CIRCULAR IN CROSS SECTION WITH THE TOTALCROSS SECTIONAL AREA OF THE INDIVIDUAL INTERSTICES BEING ESSENTIALLYCONSTANT ALONG THEIR INWARD LENGTH.